1. Field of the Invention
The present invention relates to the fabrication of printed circuit boards and more particularly to a fabrication process for forming a hermetic seal for a printed circuit board that provides a defect-free surface.
2. Description of the Prior Art
U.S. Pat. No. 3,346,950, issued Oct. 17, 1967 to Schick entitled METHOD OF MAKING THROUGH-CONNECTIONS BY CONTROLLED PUNCTURES, discloses a method and means for establishing small area through connections, between conductors disposed on opposite faces of an insulator. At desired points of interconnection, a small area of a conductive line element on one surface of an insulator body is depressed into and through the adjacent insulator material into contact with a point on a conductor on the opposite surface of the insulator body. The opposite conductor is backed by a supporting plate or the equivalent to arrest the movement of the depressing tool at its inner surface. The depressing tool is shaped to exert a stressing force on the depressed conductor to a point beyond the elastic limit of its constituent material such that worked material of this conductor is capable of withstanding the tendency of the displaced insulator material to relax into its former position after removal of the depressing tool. The connection thus formed may be electrically and structurally reinforced by plating additional conductive matter into the depressions utilizing the undeformed conductors as receiving electrodes of a plating system.
U.S. Pat. No. 3,557,446, issued Jan. 26, 1971 to Charschan entitled METHOD OF FORMING PRINTED CIRCUIT BOARD THROUGH-CONNECTIONS, describes a structure wherein a transversely deformable, conductive foil sheet is positioned over one or more holes in, and in contact with, a major surface of a nonconductive substrste. Uniform pressure is applied to the free major surface of the sheet with a hydraulic medium such as a rubber pad. Such pressure extrudes, explodes or ruptures the unsupported portion of the sheet over the hole therein to produce a through-connection.
U.S. Pat. No. 3,766,308, issued Oct. 16, 1973 to Loro entitled JOINING CONDUCTIVE ELEMENTS ON MICROELECTRONIC DEVICES, relates to a method of interconnecting adjacent conductors upon a support member, particularly in a microelectronic device. The portions of the conductors to be connected are provided with nodules or pads of malleable metal which may be deformed and spread into one another to form a cold weld useful for the interconnection of conductive tracks on integrated circuit or similar devices.
U.S. Pat. No. 4,183,137, issued Jan. 15, 1980 to Lornerson entitled METHOD FOR METALIZING HOLES IN INSULATION MATERIAL, discloses a method for metalizing the surface walls of a printed circuit board through hole to produce an electrically conductive path from one metallic layer of the board through the insulating plate to another metallic layer. A drill bit is forced through the board and into a block of soft conductor material. While the bit is turning, the conductive cuttings from the block are carried to the hole in the insulating plate and smeared on the wall surface by the bit. The smeared conductive material creates an electrically conductive path between two metallic layers of the printed circuit board.
U.S. Pat. No. 4,663,840, issued May 12, 1987 to Ubbens et al entitled METHOD OF INTERCONNECTING CONDUCTORS OF DIFFERENT LAYERS OF A MULTILAYER PRINTED CIRCUIT BOARD, relates to a method of interconnecting conductors of different layers of a multilayer printed circuit board by forming a depression in the relevant conductor of the outermost layer so that the insulation layer(s) is deformed and contact is made between the outermost layer and the appropriate conductor(s) of the next layer or layers. The method can be used in multilayer printed circuits on a substrate and in flexible multilayer printed circuits on an electrically insulating foil. In the latter case, a temporary backing surface is required for the depression process. The substrate or the temporary backing surface must be deformable to a sufficient degree.